1. Field of the Invention
The present invention relates to an electric power steering apparatus that uses electric power to assist the force required to steer a vehicle.
2. Description of the Related Art
There has been developed an electric power steering apparatus in which a vehicle speed sensor is used to detect vehicle speed and a torque sensor to detect the steering torque applied at the steering wheel, and in which, when the applied steering torque is detected being outside a prescribed dead zone, a driving current of the magnitude determined according to the detected steering torque and vehicle speed is supplied to drive a steering assisting motor whose rotational force is used to assist the force required to steer the vehicle, thereby providing a comfortable steering feel to the driver.
The steering assisting power is controlled on the basis of the results of the operations performed by a controller such as a CPU. In order to ensure the system safety against malfunctions, such as runaway, of the CPU, there has been proposed, for example, a technique of dual-CPU configuration in which the system is constructed using two CPUs, i.e. a main CPU and a sub CPU that operate with separate power supply circuits.
In electric power steering apparatus employing such a dual-CPU configuration, the main CPU calculates the value of the driving current for the motor on the basis of the detected steering torque and vehicle speed, thereby controlling the steering assisting power; on the other hand, the sub CPU performs the same operations as the main CPU does, and the results of the operations are compared with those obtained from the main CPU, thereby monitoring the inputs/outputs to and from the main CPU for the detection of a failure in the main CPU.
If the sub CPU detects a failure in the main CPU, the sub CPU performs control that involves deactivating the motor, performing a fail-safe operation, etc. to protect the system safety.
In the electric power steering apparatus employing the dual-CPU configuration as described above, the basic function of the dual-CPU system is such that the main CPU controls the steering assisting power while the sub CPU monitors the operation of the main CPU. In addition to this basic function, the dual-CPU system has the function that the main and sub CPUs mutually monitor the operation of each other by transferring watchdog pulses between them, each CPU monitoring the watchdog pulses sent from the other CPU.
However, in the above prior art electric power steering apparatus, while the system safely can be protected by the sub CPU against failures, such as runaway, of the main CPU, the main CPU would be left unmonitored in the case of run-away of the sub CPU, and the system safely cannot be secured if the two CPUs have gone out of control, because of exposure to a strong electric field for example.
Furthermore, since the main and sub CPUs between which watchdog pulses are transferred operate with separate power supply circuits, the prior art configuration has involved the possibility that one CPU may cause a latch-up of the other CPU when the supply voltages are different, which results in unstable operation.